While house hunting in late 2008, Darcy Miller Nussbaum, the editorial director of Martha Stewart Weddings magazine, took a look at a very large, very raw space that resembled something on the outer fringes of Kosovo rather than on a prime corner of Manhattan’s Upper East Side. Not long before, the concrete sprawl had been two prewar apartments, one atop the other; the previous owner had bought both with the idea of creating a grand duplex. He did the demolition, but then, having lost interest (or principal), he put the combined unit back on the market.

Darcy was unequivocably in love. There was just a little problem.

“I was the only one,” she says three years later, sitting at the family’s oval kitchen table, surrounded by former skeptics and several scrapbooks chronicling the apartment’s transformation. “Not my husband, not my mother, not my architect.”

“It was a disaster,” says Andrew Nussbaum, a lawyer, fixing himself a drink at the nearby counter.

“Way too dark,” says Madge Miller from her seat on the banquette, shaking her head.

“I said she could probably do better in terms of space and natural light,” says David Mann of MR Architecture + Decor, choosing his words carefully.

So Darcy searched for something more conventionally appealing. But almost a year later, with the cavernous construction site still on the market, she decided the second-guessing was over. In its place, a statement: “We’ll take it.”

Having watched her family grow into and then out of a smaller place a block away, Darcy knew what they needed most was square footage.

“When we moved into our last apartment, both Andy and I had our own offices,” she explains. “But when the first baby came, I lost mine, and when the second one came, Andy lost his.”

Darcy not only wanted to regain work space for herself and her husband, she also asked that each of their daughters, ages three, six, and eight, get her own room. By the time Mann and his team’s plan was carried out, the duplex was as beautifully functional as a prewar maisonette or even a house in the country. On the lower floor are a soaring entrance hall, a living/dining room, a large open kitchen, and, as promised, two work spaces. Upstairs are a generously scaled master suite and a playroom with three adjacent bedrooms for the girls. The only thing missing is a yard.

All this might suggest an apartment crafted to address strictly pragmatic concerns, but the dwelling is also an exercise in self-assured, sophisticated style.

“Darcy has a certain exuberance,” Mann says. “She does what she loves and gets great joy out of it, and that’s really echoed in how she dresses and moves around in the world. There’s an unexpected luxury and glamour about her.”

Mann, who is known for his fresh, eclectic take on modernism, had worked with Darcy on her family’s prior home a decade earlier. This time, in a nod to the decidedly unmodernistic Rococo period, when gilded surfaces served to amplify candle flames, the architect let metallic finishes help make the most of the apartment’s available light. The Venetian-plaster walls in the entrance hall, for instance, glimmer throughout the day and evening with ground mica; mirrored in the polished onyx floor and stairs are a bespoke antiqued-brass-and-silver railing, a patinated-bronze Hervé Van der Straeten light fixture, and a sleek bench upholstered in a silvery leather.

The spacious living/dining room is just as ball gown–worthy. A deep custom-made sofa is covered in a pewter-tone velvet, the Art Deco–style dining chairs in golden silk and leather. A wall of cabinets is surfaced in smoked-bronze mirror, and the fireplace is fronted with gold-leafed glass tile. Mann and his chief associate on the project, William Clukies, ensured that reflective materials wink slyly all around, from the stainless-steel cabinetry in the kitchen to the platinum glass-mosaic tile in the master bath.

The success of the duplex is a testament to both Darcy’s astuteness in seeing how well the space could accommodate her family and Mann’s dogged efforts at fine-tuning each and every wall treatment, light fixture, building material, and finish to the same pleasing note. As opulent as it all looks, however, it never feels over-the-top. “I tend to be thought of as extremely subtle,” the architect says, “which may be why this place works so well.”

And how many apartments in the neighborhood could possibly feel as conducive to entertaining as this one? Which, as it happens, Darcy is very fond of doing. Not long after she was unpacked, she hosted a bridal shower for her friend Dylan Lauren.

“It’s good to have a party as soon as you move in,” Darcy says with conviction. “It forces you to get everything done and pull it all together.”

Mann appreciates her audacity. His only wish? That she would entertain even more. “Every time Darcy has a party,” he says, “we get a new client.”

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